Alkali silicate glass

ABSTRACT

Alkali silicate glasses are described which, in view of their good chemical stability and their optical properties and processing properties, are particularly suitable as a coating or veneering material for ceramic dental frameworks and hence for the production of all-ceramic dental restorations such as crowns or bridges.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/063,236, filed Oct. 23, 1997.

This application is a continuation of U.S. application Ser. No.09/097,189, filed Jun. 12, 1998, now U.S. Pat. No. 6,121,175.

The invention relates to alkali silicate glass and, in particular, tosuch a glass which is suitable for adjusting in a desired manner theoptical properties and processing properties of coating and veneeringmaterial for ceramic dental restorations.

In addition to metallic dental restorations which are veneered withceramic layers for aesthetic reasons, all-ceramic restorations areincreasingly being used in dentistry wherein a ceramic veneering orcoating material is applied to a core of ceramic material. Inter aliaglass ceramics are suitable for use as both core and coating material.

The optical properties in particular, and the processing properties ofglass ceramic coating material are, however, often unsatisfactory. Theglass ceramics used exhibit considerable cloudiness due to their highcrystal content which is not acceptable, particularly for dentalrestorations for the incisor region. Moreover, the glass ceramics have avery high expansion coefficient in many cases, for which reason they areunsuitable as a coating material for cores of glass ceramic with a lowexpansion coefficient, such as lithium disilicate glass ceramic. As aresult of the unsatisfactory adjustment of the expansion coefficients,undesired detachment of the coating material may occur.

It is also known that leucite-containing glass ceramics in particularhave very high thermal expansion coefficients. These are attributable tothe content of leucite crystals which are formed by controlledcrystallisation of an appropriate starting glass.

Alkali silicate glasses are known from EP-A-695 726 which are suitablefor veneering primarily metallic dental frameworks and contain no B₂O₃.During heat treatment at temperatures of 600° C. to 1000° C. and henceunder conventional conditions for further dental processing, theglasses, however, form corresponding glass ceramics which, as a resultof their crystal content, are very cloudy and are therefore unsuitablefor obtaining a high translucence in a glass ceramic coating material.The crystal content, particularly leucite, also leads to undesirablyhigh expansion coefficients and sintering temperatures, so that they areunsatisfactory for veneering ceramic substrates with low expansioncoefficients.

The object of the invention is, therefore, to provide a glass which doesnot crystallise under the conventional conditions of dental processingin the temperature range from 600° C. to 1000° C., has a low thermalexpansion coefficient, a low sintering temperature, good chemicalstability and high translucence, and consequently may be added inparticular to dental glass ceramic coating material in order to improvethe properties thereof.

The alkali silicate glass according to the invention is characterised inthat it contains the following components:

Component Wt. % SiO₂ 55.0 to 71.0 Al₂O₃ 5.0 to 16.0 B₂O₃ 0.2 to 10.0 K₂O4.5 to 10.0 Na₂O 3.0 to 14.0

SiO₂ is preferably present in an amount of 55.0 to 65.0 wt. %. The glassaccording to the invention may additionally contain at least one of thefollowing components:

Component Wt. % CaO 0 to 3.0 F 0 to 3.0 P₂O₅ 0 to 0.6 Li₂O 0 to 4.0 BaO0 to 5.0 ZnO 0 to 4.0 TiO₂ + ZrO₂ 0.2 to 5.0 CeO₂ 0 to 2.0

With the exception of TiO₂ and ZrO₂, the lower limits for theseadditional components are usually 0.05 wt. %.

Preferred quantity ranges exist for the individual components of thealkali silicate glass according to the invention. These may be chosenindependently of one another and are as follows:

Component Wt. % SiO₂ 60.0 to 65.0 Al₂O₃ 6.0 to 10.0 B₂O₃ 0.5 to  8.1 K₂O5.5 to  9.0 Na₂O 3.5 to 10.0 CaO 0.5 to  3.0 F 0.2 to  2.0

Particularly preferred quantity ranges for the individual components ofthe glass according to the invention are as follows and these may bechosen independently of one another:

Component Wt. % SiO₂ 61.0 to 64.0 Al₂O₃ 7.0 to  9.0 B₂O₃ 0.5 to  4.0Na₂O 7.0 to  9.0 CaO 0.5 to  1.5 F 1.0 to  2.0 Li₂O 0 to  3.0 BaO 1.5 to 3.5 ZnO 2.0 to  3.5

All the above-mentioned quantities in wt. % relate to the glass.

For the production of the glass according to the invention, it ispreferable to proceed in such a way that suitable starting materials,such as carbonates, oxides and fluorides, are melted at temperaturesfrom 1350° C. to 1650° C., preferably 1400° C. to 1600° C. over a periodof 30 minutes to 4 hours, preferably one hour to 2.5 hours, with theformation of a homogeneous melt. The molten glass is then usuallyquenched in water i.e. fritted and, after drying, ground to the desiredparticle size.

It was possible to ascertain by scanning electron microscope analysesthat the glass according to the invention is free from crystals.Additionally, it became apparent that the glass also withstands theconditions prevailing during conventional further dental processing bysintering without the formation of crystals which occurs with knownglasses. Crystallisation did not occur even during a heat treatment attemperatures of 600° C. to 1000° C. for one minute to 2 hours.

This behaviour is presumably attributable to the special composition ofthe glass according to the invention.

The glass according to the invention usually has a sintering temperatureof 650° C. to 1150° C. Glasses having a sintering temperature of 700° C.to 1050° C. are particularly preferred. Glass which can be sintered atlow temperatures of 750° C. to 880° C. and can thus be processed isquite particularly preferred.

A rate of heating of 3 to 100° C./min and preferably 30 to 80° C./minand a holding time at the sintering temperature of 10 seconds to 1 hourand preferably 30 seconds to 5 minutes is usually chosen for carryingout sintering. It is advantageous to carry out sintering under vacuum sothat the sintered body has a few pores as possible.

The thermal expansion coefficient of the glass according to theinvention is usually 5.5 to 12.5×10⁻⁶K⁻¹, preferably 6.0 to11.0×10⁻⁶K⁻¹, measured in the temperature interval of 100° C. to 400° C.

The glass according to the invention is used by itself or together withother components preferably as dental material.

To this end it is generally used in the form of a powder with an averageparticle size of less than 90 μm. Further suitable components are glassceramics and other glasses, but also dyes, particularly colouredpigments, oxides of the 3d elements or metal colloids, and fluorescentmaterials, particularly ytterbium silicate doped with d and f elements.

Dental material which contains at least one apatite glass ceramic as thefurther component is particularly advantageous.

A preferred apatite glass ceramic is one containing CaO, P₂O₅ and F in amolar ratio of CaO:P₂O₅:F of 1:0.020 to 1.5:0.03 to 4.2 and containsapatite crystals as the main crystal phase. Such apatite glass ceramicsare characterised by particularly good chemical stability, which is ofgreat importance especially for use in dental restorations.

Moreover, the use of an apatite glass ceramic which contains at leastone of the following components and contains apatite crystals as themain crystal phase is also preferred:

Component Wt. % SiO₂ 45.0 to 70.0 Al₂O₃ 5.0 to 22.0 P₂O₅ 0.5 to  6.5 K₂O3.0 to  8.5 Na₂O 4.0 to 13.0 CaO 1.5 to 11.0 F 0.1 to  2.5

In particular preference, this apatite glass ceramic additionallycontains at least one of the following components:

Component Wt. % B₂O₃ 0 to 8.0 La₂O₃ 0 to 5.0 Li₂O 0 to 5.0 BaO 0 to 5.0MgO 0 to 5.0 ZnO 0 to 5.0 SrO 0 to 7.0 TiO₂ 0 to 4.0 ZrO₂ 0 to 4.0 CeO₂0 to 3.0

The above amounts given in wt. % relate to the apatite glass ceramic.

The apatite glass ceramics described above are produced by melting astarting glass composed of suitable starting materials, such as oxides,carbonates and fluorides, at temperatures of 1200° C. to 1650° C.,pouring this into water and subjecting the glass granules formed,optionally after further comminution, to a heat treatment attemperatures of more than 900° C. and up to 1200° C. for a period of 30minutes to 6 hours.

The apatite glass ceramics obtained are characterised by hightranslucence, good chemical stability and a low expansion coefficient.These properties are presumably attributable to their specialcomposition and to the apatite crystals produced during theirmanufacture, which crystals have in particular a needle-shapedmorphology and hence resemble the apatite crystals of natural toothmaterial.

The dental material according to the invention normally has a thermalexpansion coefficient of 5.5 to 12.5×10⁻⁶K⁻¹, measured in thetemperature range of from 100 to 400° C. The coefficient required ineach case can be adjusted by a suitable choice of the type of alkalisilicate glass and any other components and the quantities thereof.Favourable dental materials contain 10 to 90 wt. % of alkali silicateglass and 90 to 10 wt. % of other components, based on the dentalmaterial.

The dental material is suitable for coating substrates and in particularfor coating or veneering dental restorations. Coating is effected inparticular by applying the dental material to the chosen substrate andthen sintering it at temperatures of 650 to 1150° C.

In preference, a powder of the glass according to the invention is mixedwith a powder of the other components optionally present and processedto a paste by adding aqueous mixing solutions. This paste is thenapplied to a substrate and, after the desired shaping, sintering takesplace to obtain a firmly adhering coating or veneer.

The dental material according to the invention may be used as a coatingor veneering material for substrates such as dental frameworks, based onceramic or glass ceramic materials. In view of its low expansioncoefficient, it is used preferably with substrate materials with athermal expansion coefficient of 7.0 to 12.0, particularly 8.0 to11.0×10⁻⁶K⁻¹. It is used preferably for coating or veneering ZrO₂ceramics, Al₂O₃ ceramics, ZrO₂/Al₂O₃ ceramics, ceramic or glass ceramiccomposite materials and titanium.

It is used particularly advantageously, however, for veneeringsubstrates based on lithium disilicate glass ceramic in order to producein this way aesthetically very attractive fully ceramic dental productswhich have very high strength and excellent chemical stability.

Lithium disilicate glass ceramics having the following composition whichmay be obtained e.g. by melting appropriate starting glasses, fittingand heat treatment at 400° C. to 1100° C. have proved to be particularlysuitable:

Component Wt. % SiO₂ 57.0 to 80.0 Al₂O₃ 0 to  5.0 La₂O₃ 0.1 to  6.0 MgO0 to  5.0 ZnO 0 to  8.0 K₂O 0 to 13.5 Li₂O 11.0 to 19.0 P₂O₅ 0 to 11.0

with the proviso that

(a) Al₂O₃+La₂O₃ is 0.1 to 7.0 wt. % and

(b) MgO+ZnO is 0.1 to 9.0 wt. %.

The amounts given in wt. % are based on the lithium disilicate glassceramic.

For the production of coatings, dental material according to theinvention that has a thermal expansion coefficient that is smaller thanthat of the substrate to be coated is advantageous. Dental materialwhose expansion coefficient is not more than 3.0×10⁻⁶K⁻¹ smaller thanthat of the substrate is particularly advantageous.

The alkali silicate glass according to the invention and the dentalmaterial according to the invention may be processed in the usual waytogether with the additives optionally present to obtain shaped dentalproducts. Suitable shaped dental products according to the inventioncontaining the alkali silicate glass or the dental material are, inparticular, dental restorations such as an inlay, an onlay, a bridge, anabutment, a jacket, a veneer, a facet, a filling, or a connector.Particularly preferred dental restorations are bridges, crowns andpartial crowns.

The dental products preferably have a core based on ceramic or glassceramic material, particularly lithium disilicate glass ceramic, ontowhich the glass according to the invention or the dental materialaccording to the invention is applied.

Preferred lithium disilicate glass ceramics have already been describedabove.

In contrast to conventional glass, crystallisation which wouldundesirably lower its translucence does not occur with the glassaccording to the invention under the conditions prevailing during thesintering thereof. It therefore reproduces essentially the colour of thecoated substrate which is very desirable, particularly during theproduction of all-ceramic dental restorations.

The lack of crystal formation and in particular the lack of formation ofleucite crystals ascertained in the case of known glasses is aparticular advantage since the high expansion coefficient of leucitewould confer a high thermal expansion coefficient on the glass. Theglass would therefore be unsuitable for coating substrates with lowexpansion coefficients, such as ZrO₂ or lithium disilicate glassceramic. The lack of adjustment of the expansion coefficients would leadto high stresses which regularly manifest themselves in cracks andchippings. These disadvantages are not exhibited by the glass accordingto the invention due to its low expansion coefficient, so it is verysuitable for coating substrates with low expansion coefficients.

Moreover, despite its B₂O₃ content, the glass exhibits excellentchemical stability which is vital for its use as a dental material,which is permanently flushed by acid fluids in the oral cavity.

Finally, the glass may be sintered onto a substrate within a shortsintering time even at low temperatures in order to produce a firmlyadhering coating or veneer in this way.

Admixing the glass with apatite glass ceramics in particular leads todental materials which have increased translucence, a shorter sinteringtime and lower sintering temperature and a lower thermal expansioncoefficient compared with the pure apatite glass ceramic.

The invention will be explained in more detail below on the basis ofexamples.

EXAMPLES Examples 1 to 8

A total of 8 different glasses according to the invention with thechemical compositions given in Table I were produced.

TABLE 1 Compositions of glasses according to the invention (quantitiesin wt. %) Ex. No. SiO₂ Al₂O₃ P₂O₅ CaO F K₂O Na₂O Li₂O B₂O₃ TiO₂ ZrO₂CeO₂ BaO ZnO 1 56.5 6.7 0.3 3.0 0.9 8.6 6.6 1.4 4.0 — 2.5 1.0 4.7 3.8 261.5 8.7 — 1.0 1.7 7.0 8.8 — 2.4 1.5 1.0 0.5 2.9 3.0 3 60.4 11.9 — — 0.36.4 7.0 1.8 0.3 1.5 3.5 — 3.2 3.7 4 61.4 8.5 — 1.1 1.7 7.8 8.7 0.6 1.91.5 1.0 0.5 2.1 3.2 5 62.3 8.7 — 1.3 1.6 7.0 7.0 2.0 1.1 1.4 1.0 0.6 3.03.0 6 70.8 8.6 — 2.1 0.9 6.9 8.3 1.5 0.2 0.7 — — — — 7 63.4 6.2 0.4 1.7— 6.4 9.6 — 3.7 1.7 1.1 0.5 2.3 3.0 8 61.9 99 — 1.1 1.5 5.8 3.7 0.2 8.01.4 1.1 0.5 2.8 2.1

For the production of said glasses, an appropriate batch of suitableoxides, carbonates and fluorides in each case was melted in aplatinum/rhodium crucible at a temperature of 1550° C. to 1600° C. for ahomogenisation period of 1 to 1.5 hours. The glass melt was quenched inwater, and the granules of the glass formed were dried and ground to anaverage particle size of less than 90 μm.

Selected properties that were determined on specimens composed of therespective glass are given in Table II. The examples illustrate howglasses with different properties may be obtained by altering thechemical composition.

TABLE II Firing α-value tempe- x 10⁻⁶K⁻¹ Acid rature (100° C.- Opticalresistance Ex. [° C.]* Tg [° C.] 400° C.) appearance [μg/cm²] 1 760 5009.6 translucent 26 2 810 522 9.1 very 17.2 translucent 3 880 528 9.5translucent 17 4 770 494 9.4 very 26.3 translucent 5 750 468 9.4 very17.9 translucent 7 840 565 8.9 very 30 translucent 8 880 543 6.6very >100 translucent *Firing temperature = temperature which was usedfor production of the specimens by sintering onto quartz (vacuum, 1minute holding time)

Determination of the expansion coefficient α

In order to measure the thermal expansion coefficient α, a rod-shapedgreen compact was prepared from powder of the glass in question, andsaid compact was sintered in a vacuum furnace at a rate of heating of60° C./min and with a holding time of 1 minute at the respective firingtemperature. A glaze bake was then carried out without vacuum at a 20°C. higher final temperature and with a holding time of 1 minute. Thethermal expansion coefficient was determined on the specimen obtained.

Determination of acid resistance

The acid resistance is a measure of the chemical stability of glassesand glass ceramics used in dentistry in particular, since these arepermanently exposed to the action of acid substances in the oral cavity.

The acid resistance was determined according to the ISO specification6872:1995. To this end, small sample plates 12 mm in diameter and 1 mmthick were prepared initially by sintering together glass granules withan average particle size of 90 μm. The granules were kept at thesintering temperature for 1 minute. The sample plates were then treatedfor 16 hours in a Soxhlet apparatus with 4 vol. % of aqueous acetic acidand finally the loss of mass occurring was determined as a measure ofthe acid resistance.

Example 9

This Example describes the use of glasses according to the inventionaccording to Example 2 and 4 together with an apatite glass ceramic (A)as a coating material for ceramic frameworks and thus for the productionof fully ceramic dental products.

The apatite glass ceramic (A) had the composition SiO₂ 55.5 wt. %, Al₂O₃19.2 wt. %, P₂O₅ 1.2 wt. %, CaO 2.7 wt. %, F 0.6 wt. %, K₂O 6.7 wt. %,Na₂O 9.7 wt. %, B₂O₃ 0.3 wt. %, TiO₂ 1.4 wt. %, ZrO₂ 1.2 wt. % and CeO₂0.5 wt. %. For the preparation thereof, a starting glass of theappropriate composition was melted, fritted and ground to a powder. Thispowder was then heat treated for one hour at 1020° C. The crystalspresent in the glass ceramic formed could be identified as needle-shapedapatite crystals by X-ray diffractrometry.

In order to obtain a suitable expansion coefficient and sinteringtemperature, this apatite glass ceramic (A) was mixed with the alkalisilicate glasses 2 and 4 according to the invention in the form ofpowders with an average particle size of less than 90 μm and in a weightratio of 30% apatite glass ceramic (A), 35% alkali silicate glassaccording to Example 2 and 35% alkali silicate glass according toExample 4.

This mixture was sintered at 880° C. to a rod-shaped green compact in avacuum furnace at a rate of heating at 60° C./min and with a holdingtime of 1 min. A thermal expansion coefficient of 9.5×10⁻⁶K⁻¹, measuredin the temperature range of from 100 to 400° C., was determined for thesample obtained.

This mixture could thus be used for sintering onto a substrate with athermal expansion coefficient of 10.6×10⁻⁶K⁻¹, such as lithiumdisilicate glass ceramic, at an advantageous processing temperature of830+ C.

Processing on a tooth substrate can usually take place at temperaturesthat are 50 to 100° C. lower than the sintering temperature on quartz.

Example 10

In the same way as Example 9, different glasses according to theinvention may be mixed together or with other glass ceramics to obtaindesired expansion coefficients and sintering temperatures.

A powder mixture of 25 wt. % of alkali silicate glass according toExample 4 with 50 wt. % of apatite glass ceramic (B) (heat treatment at1100° C.), and 25 wt. % of apatite glass ceramic (A) according toExample 9 (heat treatment 1020° C.) was prepared in order to obtain adental material according to the invention with a low sinteringtemperature of 830° C. and an expansion coefficient of 9.5×10⁻⁶K⁻¹. Sucha material had outstanding optical properties and was highly suitable asa sintering ceramic for an all-ceramic framework structure with a lowthermal expansion coefficient.

The apatite glass ceramic (B) used in this case had the composition SiO₂59.2 wt. %, Al₂O₃ 7.9 wt. %, P₂O₅ 3.0 wt. %, CaO 5.1 wt. %, F 0.6 wt. %,K₂O 6.8 wt. %, Na₂O 9.6 wt. %, Li₂O 0.3 wt. %, B₂O₃ 1.0 wt. %, TiO₂ 1.5wt. %, ZrO₂ 2.5 wt. %, CeO₂ 0.5 wt. % and ZnO₂ 2.0 wt. %. For thepreparation thereof, a starting glass of the appropriate composition wasmelted, fritted and ground to a powder. This powder was then heattreated at 1100° C. in order to form the glass ceramic.

Examples 11 to 14

In these Examples, other mixtures of alkali silicate glasses accordingto the invention with apatite glass ceramics were examined, which arehighly suitable as coating or veneering materials that can be sinteredonto substrates with low thermal expansion coefficients.

The following apatite glass ceramics were used:

1. Apatite glass ceramic (B) according to Example 10

2. Apatite glass ceramic (C) having the composition:

SiO₂ 62.8 wt. %, Al₂O₃ 13.1 wt. %, P₂O₅ 1.2 wt. %, CaO 2.7 wt. %, F 0.6wt. %, K₂O 6.3 wt. %, Na₂O 5.9 wt. %, ZrO₂ 1.7 wt. %, CeO₂ 0.5 wt. %,BaO 1.8 wt. % and ZnO 3.4 wt. %.

3. Apatite glass ceramic (D) having the composition:

SiO₂ 64.5 wt. %, Al₂O₃ 8.4 wt. %, P₂O₅ 1.1 wt. %, CaO 2.8 wt. %, F 0.7wt. %, K₂O 6.6 wt. %, Na₂O 9.6 wt. %, B₂O₃ 2.2 wt. %, TiO₂ 1.2 wt. %,ZrO₂ 0.4 wt. %, and ZnO 2.5 wt. %.

The compositions of the individual mixtures and the heat treatmentcarried out for the production of the apatite glass ceramic used in eachcase are given in Table III.

The properties determined for these mixtures are also stated in TableIII and they show that, by means of a suitable choice of components, itis possible to obtain dental materials with properties adjusted to theapplication in question.

TABLE III Properties of mixtures of glasses according to the inventionand apatite glass ceramics α-value × Heat Sintering 10⁻⁶ K⁻¹ Acidtreatment Mixing ratio temp. Tg (100° C.- Optical resistance Ex.Composition [° C./h] [in wt. %] [° C.] [° C.] 400° C.) appearance[μg/cm²] 11 Apatite glass ceramic 1050/1 50 850 530 9.3 milky, cloudy,<100 (B) — 50 translucent Alkali silicate glass 2 12 Apatite glassceramic 1020/1 50 870 542 8.0 milky, <100 (B) — 50 translucent Alkalisilicate glass 8 13 Apatite glass ceramic 1000/1 40 910 552 8.8 very<100 (C) — 60 translucent Alkali silicate glass 5 14 Apatite glassceramic 1050/1 70 850 539 8.7 slightly milky, <100 (D) — 30 slightlyopal, Alkali silicate glass 6 translucent

What is claimed is:
 1. An alkali silicate glass, which comprises thefollowing components: Component Wt. % SiO₂ 55.0 to 71.0 Al₂O₃  5.0 to16.0 B₂O₃  0.2 to 10.0 K₂O  4.5 to 10.0 Na₂O  3.0 to 14.0 TiO₂ + ZrO₂ 0.2 to 5.0.


2. A glass according to claim 1, which additionally comprises at leastone of the following components: Component Wt. % CaO 0 to 3.0 F 0 to 3.0P₂O₅ 0 to 0.6 Li₂O 0 to 4.0 BaO 0 to 5.0 ZnO 0 to 4.0 CeO₂ 0 to 2.0.


3. A glass according to claim 1, wherein quantities of some components,independently of one another, are as follows: Component Wt. % SiO₂ 60.0to 65.0 Al₂O₃ 6.0 to 10.0 B₂O₃ 0.5 to  8.1 K₂O 5.5 to  9.0 Na₂O 3.5 to10.0;

and further comprises the following components: CaO 0.5 to 3.0 F 0.2 to2.0.


4. A glass according to claim 1, wherein the quantities of somecomponents, independently of one another, are as follows: Component Wt.% SiO₂ 61.0 to 64.0 Al₂O₃ 7.0 to  9.0 B₂O₃ 0.5 to  4.0 Na₂O 7.0 to  9.0;

and further comprises the following components: CaO 0.5 to 1.5 F 1.0 to2.0 Li₂O 0 to 3.0 BaO 1.5 to 3.5 ZnO 2.0 to 3.5.


5. A glass according to claim 1, which does not crystallise during aheat treatment at temperatures from 600° C. to 1000° C. for one minuteto 2 hours.
 6. A glass according to claim 1, which has a sinteringtemperature of 650° C. to 1150° C.
 7. A dental material, which comprisesthe glass according to claim
 1. 8. A dental material according to claim7, which has a linear thermal expansion coefficient of 5.5 to12.5×10⁻⁶K⁻¹, measured in the temperature range of from 100 to 400° C.9. A shaped dental product, which comprises the alkali silicate glassaccording to claim
 1. 10. A shaped dental product according to claim 9,which is a dental restoration.
 11. A shaped dental product according toclaim 9, which comprises a core comprising a ceramic or glass ceramicmaterial and a coating applied to the core, wherein the coatingcomprises the alkali silicate glass.
 12. A method of coating a substratecomprising: providing a substrate and coating the substrate with thedental material of claim
 7. 13. A method according to claim 12, whereinthe substrate is comprised of a ceramic or a glass ceramic.
 14. A methodaccording to claim 12, further comprising: sintering the coatedsubstrate at temperatures of 650° C. to 1050° C.